Improved oxidation process for the production of pigmentary white metal oxides

ABSTRACT

There are various methods for the production of white metal oxides such as titanium dioxide in which one or more gas or vapor are heated by passage through an electric arc or electric discharge device. The electrodes have been found to introduce significant impurities into the final product. It has now been found, however, that the discoloring impurities can be very substantially reduced by having at least one of the electrodes and preferably all of the electrodes constructed from a metal such as aluminum, titanium, zirconium, or the alloys thereof or from the electrically conducting nitrides or carbides thereof. Since the impurity results from the oxidation of the electrode, it is essential that the oxides be essentially white or colorless.

United States Patent Inventors Priority Kenneth Arkless Durham;

Denis Cleaver, Saltburn; Alan Lawrence Hare, Durham, allot England Dec.18, 1964 Nov. 2, I97 1 British Titan Products Company Limited Durham,England Dec. 24, 1963 Great Britain IMPROVED OXIDATION PROCESS FOR THEPRODUCTION OF PIGMENTARY WHITE METAL OXIDES 18 Claims, No Drawings U.S.Cl 23/202, 23/1, 23/139, 23/140, 23/142, 23/182, 106/300, 204/164 Int.Cl C01g1/02, COlg 23/04, C09c 1/36 Field 01 Search 23/202, 1, 142, 139,140, 182,183, 145,200, 153; 106/300; 204/164 Primary Examiner EdwardStern Attorney-Birch, Swindler, McKie & Beckett ABSTRACT: There arevarious methods for the production of white metal oxides such astitanium dioxide in which one or more gas or vapor are heated by passagethrough an electric are or electric discharge device. The electrodeshave been found to introduce significant impurities into the finalproduct. It has now been found, however, that the discoloring impuritiescan be very substantially reduced by having at least one of theelectrodes and preferably all of the electrodes constructed from a metalsuch as aluminum, titanium, zirconium, or the alloys thereof or from theelectrically conducting nitrides or carbides thereof. Since the impurityresults from the oxidation of the electrode, it is essential that theoxides be essentially white or colorless.

IMPROVED OXIDATION PROCESS FOR THE PRODUCTION OF PIGMENTARY WHITE METALOXIDES The present invention relates to an improved process andapparatus for the production of finely divided white metal oxide by theoxidation, in the vapor phase, of a metal-halide.

In British application No. 4609/62, published as British Pat;specification 1,035,191, there is described and claimed a process. forthe production of metal oxides, including pigmentarytitanium dioxide, bythe oxidation in the vapor phase of the corresponding halide in areaction zone into which there is introduced a gas which has been heatedby passage through an electric arc or by inducting heating. The gas soheated maybe all, or part, of one of the reactants or it may be a gaswhich is substantially inert to the reaction, such as argon or nitrogen.

One device for heating the gas whichis described in the aforesaidBritish Pat. specification is that generally known as'a plasma gun,"comprising a central cathode connected to a source of electrical energyand a cylindrical anode also connected to a source of electrical energy.The anode and/or cathode may be cooled by contact with cooling fluid.

In British application No. 3528/62now British Pat. No.

991,318, there is described and claimed a process for the production offinely divided metal oxides, including pigmentary titanium dioxide,wherein a stream of hot gas containinginitial solid particles of smalleraverage particle size than that of l the metal oxide to be produced isintroduced into areaction zone; a metal halide, for example titaniumtetrachloride, an an oxygenating gas, for example oxygen or anoxygen-containing gas mixture, are introduced into the reaction zone, atleast one of these reactants being introduced through a plurality ofin-' lets spaced along the length of the zone in the direction of thegas flow; and the temperature in the reaction zone is such that thehalide and the oxygenating gas will react to fonn metal oxide. Such aprocess is of particular value in the production of high qualitypigmentary titanium dioxide.

One method which maybe used-to produce the hot gas stream containing theinitial solid particles is passing'an inert gas through, or in contactwith, an electric are or discharge, as previously described, andintroducing a metal halide and oxygenating gas'into the hot gas streamto produce particles of metal oxide. If desired, however, all or part ofone of the reactants may be heated by contact with the electric arc ordischarge to form the hot gas stream and the particles may be formed bythe introduction into this stream of the other reactant.

Alternatively, the initial solid particles may be formed by thevaporization and recondensation of a solid material such as a metaloxide fed into the hot gas stream.

It is, of course, also possible to produce finely divided metal oxidesby introducing into the reaction zone a hot'gas stream heated by passagethrough, or contact with, an electric are or discharge, which does notcontain solid particles and' thereafter making one or more introductionsof a metal halide" and/or'anoxygenating gas into'th'e reaction zone,(although after the first introduction of both reactants initial solidparticles may be formed and the remainder of the process then may beequated with that of the aforesaid British Pat. No. 991,381.

Again the hot gas stream may'comprise all or part of oneofthe reactantsor it may be a gas substantially inert to the reaction such as argon ornitrogen.

When one u'ses the method of the aforesaid British specificationl,035,l91, or that of British Pat. No. 991,3l8inc0tporating use of anelectric are or discharge as described above, one frequently finds thatthe oxide product is discolored. We have found that this discolorationmaybe due to impurities originating in the material of construction ofthe anode or cathode of the electric are or discharge device andtransferred to the oxide product during its formation or presence in thereaction zone.

In the production of white metal oxides, for example pig'- mcntarytitanium dioxide, it is generally of the greatest importance that theproduct should be as white as possible and one source of color which hasa marked detrimental effect upon the pigment is a metal which formscolored compounds, particularly colored oxides. Examples of such metalsare iron, copper, tungsten, chromium, nickel, cobalt and vanadium.

It is an object of the present invention to provide a method for theproduction of white metal oxides by the oxidation of a metal halide, forexample by the processes described in British Pat. specification1,035,191 or the above embodiment of British Pat. No. 991,318, whereinthe presence of color-forming impurities originating in the cathode isprevented or reduced.

It is also an object of the invention to provide an apparatus for theproduction of white metal oxides, for example by said processes, whereinthe presence of color-forming impurities originating inthe anode orcathode isprevented or reduced.

Accordingly, the present invention is a process for the production offinely divided white metal'oxideby the vapor phase oxidation of a metalhalide with an oxygenating gas in a reaction zone into which isintroduced a gas which has been heated by contact with an electric areor discharge formed between a cathode and anode, at least one of thecathode and anode- (and preferably both) being formed from metal orelectrically conducting nonmetal, such metal or nonmetal giving riseonly to substantially noncoloringproducts in the reaction zone.

The term metal" as used in the specification includes silicon.

The preferred halides which are to be oxidized are chlorides. Fluoridesare unsuitable for use in'the present invention and the term halide(s)"as used in the specification does not include-iluoride( s).

A particularly useful application of the invention is in the vapor phaseoxidation of titanium halides, preferably titanium tetrachloride, totitanium dioxide, particularly pigmentary titanium dioxide.

The invention-is also of value in the production of'sil ica, zirconia oralumina by the oxidation of the appropriate halide of the metal;important examples are the oxidation of silicon tetrachlorideorzirconiumtetrachloride to produce silica or zirconia, respectively, and the 7oxidation of aluminum trichloride to p'r'oduce alumina.

Theoxygenating gas is preferably oxygen or an oxygen-containing gasmixture such as air although other gases may be used which oxidize themetal halide to the corresponding oxide under the conditions existing inthereaction zone provided, of course; that they have no adverse effectupon the reaction and/0r theproduct.

The gas heated'by contact with the electric are or discharge may beaninert gas. Alternatively, it may be the oxygenating gas or, ifdesired; it may be the metal halide.

The temperatures in the reaction zone are desirably maintained at'asufficiently high level to ensurethat the'metal hause is rapidly;oxidized to the corresponding oxide. Thus, in the case of titaniumhalides, mean reaction temperatures in the range 700 C. to 1,600 C. arepreferred, particularly mean reaction temperatures in the range 900 C.to l,300 C.

The reaction zone should be providedwith an exit port or ports throughwhich the gaseous suspension of oxide particles can be withdrawnjforrecovery and an injection port or ports for the introduction of the gasheated by contact with the electric are or discharge, themetal halideand/or'the oxygenating gas and, if desired; other additions. ln the caseof titanium halides, examples ofsuch other additions are water, aluminumtrihalide, zirconium tetrahalide, silicon tetrahalide and/or solidparticles which may act as nuclei forgrowth of titanium dioxide(suchasthe'inert solid particles of British Pat. No. 991,318), ortitaniumtrichloride.

1n the case of some metal halides, particularly titanium halides, theremay be'a tendency for some of the metal oxide formed to accumulate onthe walls in the vicinity of the reaction zone. It is, thereforepreferred, in the case of such metal titanium halides, to provide ascraping device within the reaction zone which operates in such a waythat excessive accumulations of metal oxide on the walls of the reactionzone are prevented. If desired, this scraping device may alsoincorporate means to inject the metal halide and/or the oxygenating gasinto the reaction zone. Such a device for titanium halides is describedin British application No. 49126/63 now British Pat. specificationl,097,762 published Jan. 3, 1968.

The gas which is to be heated before introduction into the reaction zonemay be heated in any device in which it is possible to maintain a stableelectric arc or discharge over a sufficiently prolonged period. Forexample, it may be heated in the "plasma gun device described in BritishPat. specification l,035,l91 in which the discharge is maintainedbetween a central cooled cathode and a surrounding cooled anode.Provision is made for the introduction of the gas around the cathode andits withdrawal through the center of the anode after passing through thearc formed between cathode and anode.

Alternatively, the device may comprise coaxial tubes of electricallyconducting material placed end to end and separated by a coaxial tube ofelectrically insulating material. One tube forms the cathode and theother the anode and each is preferably fluid cooled.

ln previously used apparatus for heating gases to a high temperature bycontact with an electric are or discharge, the electrodes are made ofmaterial selected for their good electrical and thermal properties. Purecopper is a very suitable material for many purposes but it is lesssuitable for use as an electrode when heating a gas for introducing intoa reaction zone in which a metal halide is being oxidized to producepigmentary titanium dioxide or other white metal oxide. It has beenfound that erosion of the copper may take place and the copper removedfrom the electrode contaminates the oxide product and forms a coloringimpurity with a resulting deterioration of the quality, particularly adeterioration in the brightness of the pigment in the case of theproduction of pigmentary titanium dioxide.

Other materials which may cause similar deterioration in white oxidequality when used as or in an electrode are iron, cobalt, chromium,vanadium, nickel and tungsten.

It is preferred to use a material for both the cathode and anode whichdoes not give rise to color-forming impurities in the product. In somemethods of operating the gas-heating device, for example at a highvoltage, the cathode may be the electrode which suffers greater erosion.

It is believed that the material eroded from the anode and/or cathodeforms the oxide of the material under the conditions in the reactionzone and it is generally preferred, therefore, to form the anode and/orcathode of a material which gives rise to a noncoloring oxide.

Examples of materials which give rise to noncoloring impurities in awhite oxide product and which may be used in the present process aremany metals, for example, aluminum, titanium, zirconium and alloysthereof, and the corresponding electrically conducting nonmetals whichmay be compounds of such metals, for example the carbides or nitrides(which form white oxides).

Owing to the high temperatures reached in the gas-heating device it isdesirable to provide an adequate supply of a cooling fluid, for example.water, in heat exchange contact with the anode and cathode duringoperation.

The following examples show various embodiments of the presentinvention.

EXAMPLE I.

A "plasma gun of the type described was set up consisting of a centralcathode of thoriated tungsten and an annular anode of titaniumsurrounding the front part of the cathode. The anode and cathode wereprovided with interior channels through which water was to circulate inheat exchange contact therewith.

Provision was made for the gas to be heated to be introduced behind theanode in such a manner that the gas followed a helical path in passingbetween the cathode and anode and out through the central space in theanode to the reactor.

An annular slot was formed on the external face of the anode surroundingthe exit of the heated gases. The slot was formed between two discs ofrefractory material spaced apart and fixed to the face of the anode andprovision was made for the supply of premixed titanium tetrachloride.oxygen, aluminum chloride and silicon tetrachloride to the annular slot.

The device was downwardly mounted upon a vertical silica tube 3 feet inlength. From the lower end of the tube a collection system forrecovering the pigmentary titanium dioxide was fitted. To operate theprocess the flow of coolant was commenced, argon was admitted into thedevice at a rate of 25 liters/minute and an arc was struck between thecathode and anode. The requirements of the arc were 20 volts at 350amps.

A prefixed feed of titanium tetrachloride (1 gm. mole/min.) oxygen (l.5gm. mole/min.) and sufficient aluminum trichloride to give 2 percent byweight of alumina (on TiO, produced) and sufficient silicontetrachloride to give 0.25 percent silica (on TiO, produced) wassupplied to the annular slot.

Pigmentary titanium dioxide was formed in the silica tube and wasrecovered from the collection system and was analyzed for content of thematerial from which the anode and cathode were made, and the brightnessof the product was estimated. The results of these estimations are givenin the table.

The brightness of the pigmentary titanium dioxide was estimated visuallyagainst pure magnesium oxide by means of an arbitrary scale in which themagnesium oxide was given a value of l and higher values representincreasingly inferior brightness.

EXAMPLE ll.

The process of example I was repeated using nitrogen as the heated gas.The requirements of the arc in this case were 40 volts and 200 amps.

EXAMPLE Ill.

The process of example I was repeated using an anode of zirconium metal.

EXAMPLE lV.

The process of example I was repeated using a titanium anode andthoriated tungsten cathode but argon was introduced around the cathode(as described in example V) at a rate of 5 liters/minute and oxygen wasintroduced in the manner of the argon introduction described in exampleI at a rate of 25 liters/minute. The amount of oxygen introduced throughthe annular slot on the face of the anode was correspondingly reduced.

The are requirements were 38 volts and 250 amps.

EXAMPLE V.

The process of example I was repeated using an aluminum anode andthoriated tungsten cathode.

Argon was introduced into the device at a rate of 50 liters/minute andthe arc requirements were 25 volts and 250 amps.

EXAMPLE Vl.

An arc heating device of different design was used. This consisted oftwo jacketed aluminum tubes of one-half inch internal diameter and Hiinch long. These were placed coaxially, the adjacent ends beingseparated by a tubular insulating body 2 inches in length. Provision wasmade for the introduction of part of the gas to be heated into one endof the device and the remainder was introduced tangentially through thewall of the insulated body. The first aluminum tube (into the end ofwhich part of the gas was introduced) was the cathode and the tube onthe opposite side of the insulating body (and from which the heated gasissued) was the anode.

The remainder of the apparatus was as described in example I.

Water was circulated through the jackets surrounding the anode andcathode and I liters/minute of oxygen were introduced into the open endof the anode and 40 liters/minute were introduced through the wall ofthe insulating body.

The are requirements were 300 volts and 80 amps under these conditions.

Titanium tetrachloride 3 gm mole/minute premixed with 60 liters/minuteoxygen and sufficient aluminum chloride and silicon tetrachloride togive 2 percent alumina and 0.25 percent silica (by weight on TiO werefed to the annular slot on the face of the device.

EXAMPLE VII.

The processes of examples l and II were repeated in a manner not inaccordance with the present invention using copper anodes. Analysis andbrightness of the pigmentary titanium dioxide produced are given in thetable.

The are requirements given in the examples are from a source of directcurrent but a source of alternating current may be used but is generallynot preferred.

The flow rates in the examples given as liters/minute are measured atroom temperature and pressure.

10 elevated temperature, the improvement which comprises heating a gasby passage through the plasma arc discharge of at least two electrodesand then feeding the gas to the reaction zone, at least one of saidelectrodes constructed of zirconium carbide.

II. In a process for the production of finely divided pigmentary whitemetal oxide by the oxidation of at least one metal halide in the vaporphase in a reaction zone maintained at an elevated temperature, theimprovement which comprises heating a gas by passage through the plasmaarc discharge of at least two electrodes and then feeding the gas to thereaction zone, at least one of said electrodes constructed of aluminumnitride.

12. In a process for the production of finely divided pigmentary whitemetal oxide by the oxidation of at least one metal halide in the vaporphase in a reaction zone maintained at an elevated temperature, theimprovement which comprises heating a gas by passage through the plasmaarc discharge of Impurities from Impurities Brightness anode (p.p.m. onfrom of pigmen- Example Anode material Cathode material Gas heated TiOz)cathode 1 tary 'IiO;

rgon 2 2 (zirconium) 1-2 2 Thoriated tungsten Argon (copper)... I-Z 8VIII do do Nitrogen 100 (copper) 3 12 1 Parts per million of W0 on TiOWhat is claimed is:

I. In a process for the production of finely divided pigmentary whitemetal oxide by the oxidation of at least one metal halide in the vaporphase in a reaction zone maintained at an elevated temperature, theimprovement which comprises heating a gas by passage through the plasmaarc discharge of at least two electrodes and then feeding the gas to thereaction zone at least one of said electrodes constructed of a materialselected from the group consisting of metals which form substantiallynoncoloring products on reaction with the contents of said reactionzone, and the alloys and electrically conducting carbides and nitridesthereof.

2. The process of claim I wherein the metal halide is titaniumtetrachloride and said titanium tetrachloride is oxidized by anoxygen-containing gas.

3. Th e process of claim 2 wherein the reaction zone is maintained at atemperature of 700 to 1,600 C.

4. The process of claim 1 wherein the gas heated by passage through theplasma arc is an inert gas.

5. The process of claim 1 wherein the gas heated by passage through theplasma arc is an oxygenating gas.

6. The process of claim 1 wherein the gas heated by passage through theplasma arc is a metal halide.

7. The process of claim 1 in which the electric arc is radially in linewith the reaction zone.

8. In a process for the production of finely divided pigmentary whitemetal oxide by the oxidation of at least one metal halide in t hvaporphase in a reaction zone maintained at an elevated temperature, theimprovement which comprises heating a gas by passage through the plasmaarc discharge of at least two electrodes and then feeding the gas to thereaction zone, at least one of said electrodes constructed of aluminumcarbide.

9. In a process for the production of finely divided pigmentary whitemetal oxide by the oxidation of at least one metal I I. In a process fortheproduction finely pigmentary white metal oxide by the oxidation of atleast one metal halide in the vapor phase in a reaction zone maintainedat an elevated temperature, the improvement which comprises heating agas by passage through the plasma arc discharge of at least twoelectrodes and then feeding the gas to the reaction zone, at least oneof said electrodes constructed of aluminum.

lfii zTprocess for the productionof finely divided pigmentary whitemetal oxide by the oxidation of at least one metal halide in the vaporphase in a reaction zone maintained at an elevated temperature, theimprovement which comprises heating a gas by passage through the plasmaarc discharge of at least two electrodes and then feeding the gas to thereaction zone, at least one of said electrodes constructed of titanium.

16. In a process for the production of finely divided pigme n tary whitemetal oxide by the oxidation of at least one metal halide in the vaporphase in a reaction zone maintained at an elevated temperature, theimprovement which comprises heating a gas by passage through the plasmaarc discharge of at least two electrodes and then feeding the gas to thereaction zone, at least one of said electrodes constructed of zirconium.17. In a process for the production of finely divided pigmentary whitemetal oxide by the oxidation of at least one metal halide in the vaporphase in a reaction zone maintained at an a t leastastoichiometijcamountof an oxygen-containing gas in a reactionzoneat elevated temperatures, the improvement which comprises generatinga gaseous plasma by passing a gaseous stream through a discharge ofelectrical energy. said discharge being conducted between electrodes atleast one of which contains a white oxide forming metallic element,

generating from such electrode under the conditions of said electricaldischarge particles comprising said metallic ele- ;ment, and forwardingsaid gaseous stream to said reaction zone whereby to carry particlescomprising said metallic element into said reaction zone and intocontact with said metal halide and oxygen-containing gas.

i i k I I!

2. The process of claim 1 wherein the metal halide is titaniumtetrachloride and said titanium tetrachloride is oxidized by anoxygen-containing gas.
 3. The process of claim 2 wherein the reactionzone is maintained at a temperature of 700* to 1,600* C.
 4. The processof claim 1 wherein the gas heated by passage through the plasma arc isan inert gas.
 5. The process of claim 1 wherein the gas heated bypassage through the plasma arc is an oxygenating gas.
 6. The procesS ofclaim 1 wherein the gas heated by passage through the plasma arc is ametal halide.
 7. The process of claim 1 in which the electric arc isradially in line with the reaction zone.
 8. In a process for theproduction of finely divided pigmentary white metal oxide by theoxidation of at least one metal halide in the vapor phase in a reactionzone maintained at an elevated temperature, the improvement whichcomprises heating a gas by passage through the plasma arc discharge ofat least two electrodes and then feeding the gas to the reaction zone,at least one of said electrodes constructed of aluminum carbide.
 9. In aprocess for the production of finely divided pigmentary white metaloxide by the oxidation of at least one metal halide in the vapor phasein a reaction zone maintained at an elevated temperature, theimprovement which comprises heating a gas by passage through the plasmaarc discharge of at least two electrodes and then feeding the gas to thereaction zone, at least one of said electrodes constructed of titaniumcarbide.
 10. In a process for the production of finely dividedpigmentary white metal oxide by the oxidation of at least one metalhalide in the vapor phase in a reaction zone maintained at an elevatedtemperature, the improvement which comprises heating a gas by passagethrough the plasma arc discharge of at least two electrodes and thenfeeding the gas to the reaction zone, at least one of said electrodesconstructed of zirconium carbide.
 11. In a process for the production offinely divided pigmentary white metal oxide by the oxidation of at leastone metal halide in the vapor phase in a reaction zone maintained at anelevated temperature, the improvement which comprises heating a gas bypassage through the plasma arc discharge of at least two electrodes andthen feeding the gas to the reaction zone, at least one of saidelectrodes constructed of aluminum nitride.
 12. In a process for theproduction of finely divided pigmentary white metal oxide by theoxidation of at least one metal halide in the vapor phase in a reactionzone maintained at an elevated temperature, the improvement whichcomprises heating a gas by passage through the plasma arc discharge ofat least two electrodes and then feeding the gas to the reaction zone,at least one of said electrodes constructed of titanium nitride.
 13. Ina process for the production of finely divided pigmentary white metaloxide by the oxidation of at least one metal halide in the vapor phasein a reaction zone maintained at an elevated temperature, theimprovement which comprises heating a gas by passage through the plasmaarc discharge of at least two electrodes and then feeding the gas to thereaction zone, at least one of said electrodes constructed of zirconiumnitride.
 14. In a process for the production of finely dividedpigmentary white metal oxide by the oxidation of at least one metalhalide in the vapor phase in a reaction zone maintained at an elevatedtemperature, the improvement which comprises heating a gas by passagethrough the plasma arc discharge of at least two electrodes and thenfeeding the gas to the reaction zone, at least one of said electrodesconstructed of aluminum.
 15. In a process for the production of finelydivided pigmentary white metal oxide by the oxidation of at least onemetal halide in the vapor phase in a reaction zone maintained at anelevated temperature, the improvement which comprises heating a gas bypassage through the plasma arc discharge of at least two electrodes andthen feeding the gas to the reaction zone, at least one of saidelectrodes constructed of titanium.
 16. In a process for the productionof finely divided pigmentary white metal oxide by the oxidation of atleast one metal halide in the vapor phase in a reaction zone maintainedat an elevated temperature, the improvement which comprises heating agas by passage through the plasma arc discharge of at least twoelectrodes and then feeding the gas to the reaction zone, At least oneof said electrodes constructed of zirconium.
 17. In a process for theproduction of finely divided pigmentary white metal oxide by theoxidation of at least one metal halide in the vapor phase in a reactionzone maintained at an elevated temperature, the improvement whichcomprises heating a gas by passage through the plasma arc discharge ofat least two electrodes and then feeding the gas to the reaction zone,each of said electrodes constructed out of at least one metal selectedfrom the group consisting of aluminum, titanium, zirconium and thealloys thereof.
 18. In a process of preparing finely divided whitepigmentary metal oxide by vapor phase oxidation of a metal halide withat least a stoichiometric amount of an oxygen-containing gas in areaction zone at elevated temperatures, the improvement which comprisesgenerating a gaseous plasma by passing a gaseous stream through adischarge of electrical energy, said discharge being conducted betweenelectrodes at least one of which contains a white oxide forming metallicelement, generating from such electrode under the conditions of saidelectrical discharge particles comprising said metallic element, andforwarding said gaseous stream to said reaction zone whereby to carryparticles comprising said metallic element into said reaction zone andinto contact with said metal halide and oxygen-containing gas.